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HomeNRC LibraryDocument CollectionsGeneric CommunicationsInformation Notices1993

# Information Notice No. 93-99: Undervoltage Relay and Thermal Overload Setpoint Problems

```
                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                               December 21, 1993

NRC INFORMATION NOTICE 93-99:  UNDERVOLTAGE RELAY AND THERMAL OVERLOAD
                               SETPOINT PROBLEMS

Addressees 

All holders of operating licenses or construction permits for nuclear power
plants.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to continuing discoveries of undervoltage relay and
thermal overload setpoints that are set too low.  It is expected that
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems.  However,
suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.

Background

In August 1976, the NRC wrote generic letters to all LWR licensees regarding
the need for licensees to evaluate any generic implications of several events
at the Millstone site involving episodes of sustained degraded grid voltage. 
On June 2, 1977, as part of multiplant action MPA-23, the staff stated its
position that all licensees must have a second level of undervoltage
protection with a time delay.  (During degraded grid voltage episodes,
undervoltage relays and thermal overload protective relays protect the safety
equipment.)  In a generic letter dated August 8, 1979, the NRC requested all
licensees to determine the capability of the offsite power system to operate
all required loads within their voltage ratings under all conditions within
their design basis.  The licensees reviewed their undervoltage relay setpoints
on each of these occasions.  In 1981, the NRC issued Branch Technical
Position, Power System Branch-1, "Adequacy of Station Electric Distribution
System Voltages," which discussed, among other things, the addition of a
second undervoltage relay with an associated time delay.  Between 1981 and
1992, the NRC issued two information notices, in which it discussed
undervoltage relay setpoints and other concerns:  

      1.  IN 84-02, "Operating a Nuclear Power Plant at Voltage
          Levels Lower than Analyzed," issued January 10, 1984.

      2.  IN 91-29, "Deficiencies Identified During Electrical Distribution
          System Functional Inspections," issued April 15, 1991.

9312150073.

                                                            IN 93-99
                                                            December 21, 1993
                                                            Page 2 of 3 

      3.  IN 91-29, Supplement 1, "Deficiencies Identified During
          Electrical Distribution System Functional Inspections," issued
          September 14, 1992.

In addition, recognizing that certain degraded grid voltage problems resulted
from inadequate control over the design process, on September 12, 1988, the
NRC issued Generic Letter 88-15, entitled, "Electric Power Systems -
Inadequate Control Over Design Processes." 

Description of Circumstances

Since 1989, the NRC has performed electrical distribution system functional
inspections at nuclear power plants.  The NRC has found design weaknesses in
several electrical distribution system areas including undervoltage relay
setpoints for degraded grid voltage.  These are addressed in IN 91-29 and in
IN 91-29, Supplement 1.  From May 1990 to January 1993, over 30 licensees
wrote to the NRC about inadequate setpoints.  Licensees found that the
undervoltage relay setpoints were set so low that safety equipment would not
have been protected if degraded grid voltage had occurred.  Three licensees
discovered deficiencies in which the thermal overload protective relay
setpoints were set too low.   

Discussion

In the last few years, licensees began extensive efforts to improve the
adequacy and completeness of the set of design bases, design analyses, and
final design output documents that define the design of their facilities.  The
licensees began these initiatives primarily because, during inspections such
as safety system functional inspections and safety system outage modification
inspections, the NRC consistently found that some licensees have made plant
modifications which have affected the functionality of safety systems without
making the appropriate setpoint change.  The NRC inspection findings prompted
many licensees to review and reconstitute their design bases. 

These reviews and the generic communication documents discussed above prompted
licensees to submit technical specification changes for undervoltage relay
setpoints that were discovered to be incorrect after the problem was created
for one of several reasons shown (see Attachment 1).  Attachment 1 was
developed from 50.72 reports as a representative summary of the types and
number of problems encountered.  Subsequent LERs may provide additional
information.  Of the reports where the cause could be determined based on the
event report, design error was the predominant cause of the problem. 
Licensees have generally found setpoint problems when the setpoints were
examined as a result of a special inspection or design basis reconstitution..

                                                            IN 93-99
                                                            December 21, 1993
                                                            Page 3 of 3 

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.

                                    /S/'D BY BKGRIMES

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  C. Morris, NRR
                    (301) 504-2778

Attachments: 
1.  Representative List of Undervoltage 
      Relay Setpoint Revisions 1988-1993
2.  List of Recently Issued NRC Information Notices

.

                                                            Attachment 1
                                                            IN 93-99
                                                            December 21, 1993
                                                            Page 1 of 2

Representative List of Undervoltage Relay Set Point Revision Notifications and
            Technical Specification Change Proposals for 1988-1993

Event Notification   Licensee                 Event Notification Date  Class
   Number                                     

11374                Pilgrim 1                January 30, 1988           1
14043                Pilgrim 1                November 18, 1988          5
00000                Pilgrim 1                June 30, 1988              1

14780                Cooper 1                 February 17, 1989          5
16540                Crystal River 3          September 8, 1989          1
17121                Robinson 2               November 16, 1989          1

18322                Oconee 1, 2 and 3        April 24, 1990             5
18466                McGuire 1 and 2          May 14, 1990               1
18892                St. Lucie 2              July 14, 1990              2
19023                Haddam Neck 1            August 2, 1990             5
20021                Calvert Cliff 1 and 2    December 6, 1990           1

20435                Ft. Calhoun              February 12, 1991          1
20503                Kewaunee 1               February 20, 1991          5
20542                Salem 2                  February 27, 1991          2
21691                Dresden 2 and 3          August 23, 1991            5
22281                Dresden 2                November 20, 1991          1
22498                Zion 1 and 2             December 19, 1991          5
 
22580                Indian Point 3           January 9, 1992            5
22658                Dresden 3                January 22, 1992           5
22847 (TOL)          Washington Nuclear 2     February 19, 1992          5
22918                Washington Nuclear 2     March 1, 1992              1
23148                Vermont Yankee           April 1, 1992              4
23191                Quad Cities 1 and 2      April 7, 1992              5
23338                LaSalle 1 and 2          April 27, 1992             1
23365                LaSalle 2                April 29, 1992             1
23385                Crystal River 3          May 1, 1992                5
23439 (TOL)          Grand Gulf               May 11, 1992               3
23452                Comanche Peak 2          May 13, 1992               1
23576                Crystal River 3          June 4, 1992               1
23784                Zion 1 and 2             July 2, 1992               5
23932                Byron 1 and 2            July 24, 1992              1
00000                Callaway                 July 23, 1992              1
24229                Crystal River 3          September 14, 1992         1
24384                Dresden 2 and 3          October 6, 1992            5

*TOL = Thermal Overload.

                                                            Attachment 1
                                                            IN 93-99
                                                            December 21, 1993
                                                            Page 2 of 2

Event Notification   Licensee                 Event Notification Date  Class
   Number                                     

00000                Prairie Island 1 & 2     November 6, 1992           1
00000                St. Lucie 1              November 30, 1992          1
24757                Maine Yankee             December 15, 1992          5
00000                Diablo Canyon 1 and 2    December 22, 1992          1

24845                Point Beach 1 and 2      January 7, 1993            1
25248 (TOL)          Pilgrim                  March 29, 1993             5
25362                South Texas              April 6, 1993              5

Events Notification Number-00000 Licensees whose undervoltage relay setpoint
notifications to the NRC were reported by LERs, licensee letter, etc., and
event notification numbers if any, were not known.

Class Definitions:

1.    Undervoltage Relay setpoints were found to be incorrect because of
      design errors.  

2.    Undervoltage Relay setpoints, as found, were not the same as required by
      the technical specification.

3.    Safety load circuit breaker thermal overloads or other trip setpoints
      were too low because of design errors.

4.    Safety load circuit breaker thermal overload, or other trip setpoints,
      as found, were not the same as required by the technical specification. 

5.    Membership in one of the preceding four classes could not be definitely
      established because of incomplete information in the event notification.
.
```

*Page Last Reviewed/Updated Tuesday, November 12, 2013*

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